It is known that modern internal combustion engines are provided with a fuel injection system (FIS) for directly injecting the fuel into the cylinders of the engine. As an example, a common rail system (CRS) is a common configuration for Diesel Engines. The CRS, generally, includes a fuel pump, hydraulically connected to a fuel common rail and one or more electrically controlled fuel injectors, which are individually located in a respective cylinder of the engine and which are fluidically connected to the fuel rail through dedicated injection pipes.
The fuel pump is controlled in order to provide a fuel pump output, i.e. to supply fuel to the rail, and the at least one injector is controlled to provide an injector fuel output, i.e. to supply fuel, exiting from the rail, to the cylinder of the engine.
It has to be noted that the term “fuel output” is used herein to indicate a fuel quantity or a fuel quantity provided in an interval, thus representing a fuel flow rate. It has to be also noted that the fuel quantity provided in an interval can be referred to as a time interval, or to at least part of a cycle (or of an event), for example of the fuel pump or of the engine during its operation. As known, the fuel quantity provided can be indicated for example as a function of a stroke, or a combustion cycle, etc., thus indicating also in this case a fuel flow rate, i.e. as a fuel quantity provided in an interval.
Returning now to the fuel injection system, the rail pressure may be an important parameter for determining the quality of the fuel injection within an engine (for example, the fuel spray penetration in the cylinder head). The rail pressure must be regulated as function of the engine operating conditions. For example a target value of the fuel rail pressure can be determined according to an engine load vs. engine speed map. Thus, the fuel rail pressure is controlled in order to reach the target value of the fuel rail pressure needed in the relevant fuel injection system conditions.
The fuel rail pressure can be controlled by adjusting the fuel flow-rate (fuel quantity) pumped into the fuel rail by the fuel pump output. This adjustment of the fuel flow-rate (fuel quantity) can be determined with a sensor based feedback control. In particular, a pressure sensor detects the pressure within the fuel rail, and the detected value is compared with the fuel rail pressure target value. Subsequently, the fuel flow rate (fuel quantity) pumped into the fuel rail is adjusted in order to minimize the error between the target value of fuel rail pressure and the value of fuel rail pressure measured by the fuel rail pressure sensor.
The fuel output of the fuel pump can be adjusted in different ways. As an example, it is possible to control the electric signal driving a fuel-metering valve, usually associated to the high-pressure fuel pump, to regulate the fuel flow-rate (fuel quantity) which is supplied into the fuel rail. The fuel-metering valve may be integrated in the high-pressure fuel pump, in order to realize a single device that is usually referred as fuel metering unit. The fuel-metering valve may be a suction control valve (SCV) or a digital valve. The electric signal driving the metering valve (i.e. the signal that causes the high-pressure fuel pump to provide the required fuel pump output, or in other words to supply the required fuel flow-rate (fuel quantity)) may be an electrical current for SCVs or the timing of the electric pulses for digital valves.
As mentioned above, the fuel output of the fuel pump is determined as a function of the difference between the fuel rail pressure target value and the value of the pressure measured by the fuel rail pressure sensor. In case of failure of the rail pressure sensor, it is not possible to carry out the above mentioned feedback control. As a result, the fuel rail pressure cannot be regulated, so that the engine must be shut-down to avoid problems.